An organic electroluminescence device (hereinafter, occasionally abbreviated as organic EL device) using an organic substance is highly expected to be used as an inexpensive solid-emitting full-color display device having a large area and has been variously developed. A typical organic EL device includes an emitting layer and a pair of opposing electrodes between which the emitting layer is interposed. When an electric field is applied on both of the electrodes, electrons are injected from the cathode while holes are injected from the anode. Further, the electrons are recombined with the holes in the emitting layer to generate an excited state. When the excited state is returned to a ground state, energy is emitted as light.
The emitting layer typically contains the host material and the dopant material, where excitation is caused by the host material and light is emitted by the dopant material. Such separation of the functions of the host material and the dopant material promotes development of technology of improving a device performance.
For instance, Patent Literatures 1 to 4 disclose an organic electrical field light emission device including an organic emitting layer in which a host material, a hole-trapping dopant, and an electron-trapping dopant coexist. In the organic electrical field light emission device disclosed in Patent Literature 1, the holes injected into the organic emitting layer are trapped by the hole-trapping dopant while the electrons injected into the organic emitting layer are trapped by the electron-trapping dopant.
In the organic electrical field light emission device disclosed in Patent Literatures 1 to 3, both of the hole-trapping dopant and the electron-trapping dopant have a smaller singlet energy than the host material. In other words, in the organic electrical field light emission device disclosed in Patent Literatures 1 to 3, both of the hole-trapping dopant and the electron-trapping dopant emit light. Accordingly, the light emitted from the organic electrical field light emission device is in a color mixed with emission colors from the respective dopants and has a wide half bandwidth of an emission peak.
Moreover, in Patent Literature 4, both of the host and the hole-trapping dopant have a smaller singlet energy than the electron-trapping dopant. In other words, the hole-trapping dopant emits light. Moreover, the host is a pyrene derivative.
Recently, a further longer lifetime has been desired for a practical use of the organic EL device.